Custom Exhaust

[Drew-B6]

Ok, I am only going to be K04 at most. I am getting a custom exhaust and can't figure out if I want to 3" from CAT to Y pipe or 2.5". Please let me know what would be best and reasons why. Thanks

[dirtybrd]

2.5" then dual 2" at the Y would be plenty big. Going 3" will cause you to lose some velocity.

[Drew-B6]

(Great Article that Old Guy sent me)!!

Here is a very interesting post from last week by Tony the Tiger on Volvospeed. He is a pretty knowledgeable guy. Anyway, an interesting read:

The general rules for exhaust and downpipe sizing for a turbo car is pretty simple.. The bigger the better.

Some may think that a small turbo doesn't need a large downpipe or exhaust.. That is completely opposite of reality. With a small turbo, the pressure levels between the engine and the turbo is high, obviously due to small turbine and smaller turbocharger needing to wind itself up to 16-18 PSI of boost or higher to create any significant power. When the pressure between the turbo and the engine is high, it requires even more exhaust flow AFTER the turbine wheel. That means when you decide to run a small turbocharger at its limits, which often means lots of boost (high intake manifold pressure) and a turbine near choke line on the turbine flow maps, it is in more desperate need of a larger exhaust downstream of the turbine wheel (Downpipe/cat/exhaust/muffler) versus a larger turbocharger.

Forget about "HP" numbers.. 200 HP, 300 HP, 500 HP, these numbers are just rough figures to go with. Real exhaust sizing is determined by turbine flow vs compressor flow, exhaust manifold back pressure between engine and turbo, and back pressure after the turbine exit. I don't have the actual formula on hand to calculate pressure differential between the engine, pre- and post-turbo, but generally, each PSI of exhaust back pressure backtracking against the turbine exit equates to about 4-5 times the amount of exhaust back pressure upstream (between engine and turbo). So with a smaller turbo, it usually runs at high boost (high intake manifold pressure), which in turn elevates the exhaust back pressure between the engine and turbo already... That means any noticeable restriction AFTER the turbine exit (downpipe/exhaust/muffler) will create an exponential increase of exhaust pressure upstream between the engine/turbo, thus, choking the engine, killing performance, high EGT's and more likely to detonate. Picture the turbo's turbine side as an air pump that is designed to "suck" in exhaust flow with its blade direction, turbine housing design and passages meant to push exhaust "out" of the turbo. It's main job is to absorb the exhaust energy and flow, and then dump it out of the turbine exit, just like a vacuum cleaner! If you have a restriction at the vacuum's suction side, say you are vacuuming your carpet or something, it still manages to have its sucking power. The moment you put your hand and block off the vacuum's exit/exhaust port, it becomes useless. Same theory actually.

A more real life example would be.... Well, smaller turbos need a lot of PSI, therefore, also create a lot of exhaust back pressures for the same given HP of compressor flow as mentioned earlier. So when you have a large turbo with a huge turbine (ie: GT35R), if you run this GT35R at 300 WHP @ 12 psi for example, it actually requires a smaller downpipe and exhaust than a 19T pushing the same WHP @ 18 PSI. The GT35R has a much lower exhaust back pressure between the engine and turbo from the start, therefore, it can operate flawlessly with a smaller exhaust.

That is why you often hear factory turbo cars running their factory turbos near the limits gaining so much spool, power and response from large downpipes and exhaust. Such examples would be DSM's (Eagle Talons/Mitsu Eclipses), all Subarus, and actually almost every turbo car out there that has bolt-on upgrades to "max out" the stock turbo(s).

Larger turbos on the other hand, makes you wonder why a 600-800 WHP Supra can still run the same friggin 3" downpipe and exhaust and still have no problems because we all know Supra run ridiculously large turbines.

So a good guideline is, take a look at the turbo you are running and how close you are to maxing out the turbo. But in almost all cases, the bigger is better, but now knowing that a smaller turbo running at full tilt requires even more exhaust flow, it gets you thinking when you make your next downpipe/exhaust purchase smile.gif

Go 3" or larger, or go home! No and's if's or but's if you are looking for performance. You can easily build a 3" exhaust that is quiet by running the correct muffler and a decent resonator.

Cheers!

Old Guy
__________________
Just another silver Audi.

[texasboy21]

^^^ sounds good to me! Custom 3" downpipe but what about after the Y pipe? 3" also or 2.5" Either or probably but im considering sound, loud is what im going for (with out the classic 4banger fart or buzzz)

barrett

[AudiA4_20T]

^^^ sounds good to me! Custom 3" downpipe but what about after the Y pipe? 3" also or 2.5" Either or probably but im considering sound, loud is what im going for (with out the classic 4banger fart or buzzz)

barrett

mine is 3" split to twin 2.25"

[whiteUSPflow]

^^^Same

[dirtybrd]

Old guy is correct. But...none of these cars are running Ko3's or 4's. They run turbos around the size of a T25. 2.5" or 3", you won't notice the difference. These turbos will never out flow a 2.5" DP. I run a 3" to dual 2.5 and it's dreadfully loud and drones badly.

[Drew-B6]

See I am still confused on what to do! I would either go 3 or 2.5 from cat down to the Y and go 2.25 out the back to my 4" tips.

[old guy]

Just to be clear, the information I sent to Drew-B6 is Tony's opinion based on his experience with a 1.9T Volvo engine. Keep in mind that the 1.9T Volvo isn't set up the same as the 1.8T with the cat bolted to the turbo. I have read arguments for and against the need for a 3" down pipe following a 2½" Cat. Personally I am not convinced one way or the other that it will make a difference with the cat between the turbo and down pipe. Just passing along what I read.

Cheers!

Old Guy

[Drew-B6]

see i'm thinking if I go 3" I will still have plenty of back pressure because I am still going to keep my CAT and the bends in the pipe not being mandrel bent (I can't find anyone around here that mandrel bends)

[dirtybrd]

If you're getting crush bent done, go 3". I couldn't find anyone who does mandel bent either. I had mine tig'd together with bends.

[Condo]

I run a 3" to dual 2.5 and it's dreadfully loud and drones badly.

Mine is also 3" to dual 2.5 but it's not too loud and there is hardly any drone.

[dirtybrd]

What is your set-up?

[Drew-B6]

Right now I am running APR 1+ with OEM TT injectors and Cone filter intake with heatshield and EVOMS DV. Nothing Fancy, I was going to do a testpipe but I may just go with a HFC.

[Condo]

What is your set-up?

3" down pipe, 2.5 after the split. 2 magnaflow 14851s in the rear and 1 magnaflow 14259 before the split.

[AudiA4_20T]

3" down pipe, 2.5 after the split. 2 magnaflow 14851s in the rear and 1 magnaflow 14259 before the split.

dude what is going on with your car? i thought it drove man? 28psi is next

[dirtybrd]

I have a set of OBX SS 2.5" offset inlet center outlets I want to put on. I think that'll help alot! I miss when my car was quiet. Driving it with Tracy in it or on the highway blows!

[Condo]

dude what is going on with your car? i thought it drove man? 28psi is next

My car is running great, how's yours? Installed meth injection last week and switched to a Synapse BOV. 28psi will have to wait until I upgrade the fuel line and injectors. I'm only at 20psi for now.

[AudiA4_20T]

20psi is lameness lol. Mine is in hibernation

[Condo]

20psi is lame but it will keep it running until I can upgrade everything that needs it.

[AudiA4_20T]

20psi is lame but it will keep it running until I can upgrade everything that needs it.

fair enough. I'm in the same boat. Stock with 1000cc injectors aren't going to cut it LOL

[Drew-B6]

thanks for the off topic posts